The xylene isomers are important intermediates which find wide and varied application in chemical syntheses. Para-xylene is a feedstock for terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
The proportions of xylene isomers obtained from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20-25% of a typical C8-aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization. In selecting appropriate technology, it is desirable to obtain a ratio of aromatic isomers as close to equilibrium as practical in order to maximize the para-xylene yield; however, a close approach to equilibrium is associated with a greater cyclic C8 loss due to side reactions. The approach to equilibrium that is used is an optimized compromise between high C8 cyclic loss at high conversion (i.e. very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C8 aromatics. Catalysts thus are evaluated on the basis of a favorable balance of activity, selectivity and stability.
Catalysts containing molecular sieves have become prominent for xylene isomerization in the past quarter-century or so. U.S. Pat. No. 3,856,872, for example, teaches xylene isomerization and ethylbenzene conversion with a catalyst containing ZSM-5, -12, or -21 zeolite. U.S. Pat. No. 4,899,011 teaches isomerization of C8 aromatics using two zeolites, each of which is associated with a strong hydrogenation metal. U.S. Pat. No. 5,240,891 discloses a MgAPSO molecular sieve having a narrow ratio of framework magnesium and its use in xylene isomerization. U.S. Pat. No. 6,222,086 teaches the use of two zeolitic catalysts for the isomerization of a mixture of xylenes and ethylbenzene wherein the content of platinum-group metal in the second catalyst is no more than about 30% of that in the first catalyst. U.S. Pat. No. 6,448,459 discloses a process combination comprising recovery and isomerization of a first fraction of enriched ethylbenzene concentrate, recovery of para-xylene by adsorption from the second fraction from ethylbenzene enrichment, and isomerization of raffinate and desorbent from the para-xylene adsorption step. U.S. Pat. No. 6,660,896 teaches a process for isomerizing a feed containing ethylbenzene and a mixture of xylene isomers using first and second catalysts in the presence of hydrogen to produce a product having higher-than equilibrium para-xylene. Although these references teach individual elements of the present invention, none of the art suggests combination of the elements to obtain the critical features of the process of the present invention.
Catalysts for isomerization of C8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. The former approach commonly results in higher ethylbenzene conversion and more effective xylene isomerization, thus lowering the quantity of recycle in a loop of isomerization/para-xylene recovery and reducing concomitant processing costs, but the latter approach enhances xylene yield by forming xylenes from ethylbenzene. A catalyst system and process which combines the features of the approaches, i.e., achieves ethylbenzene isomerization to xylenes with high conversion of both ethylbenzene and xylenes, would effect significant improvements in xylene-production economics.